1. Field of the Invention
The present invention relates to a driving circuit for a flat panel display, and more particularly, to a driving circuit for a flat panel display for supplying a current of an analog signal corresponding to a size of a digital image signal to data lines of an organic electroluminescence display (OELD)
2. Discussion of the Related Art
Generally, cathode ray tubes (CRTs), which are among the widely used display devices, are typically used as monitors of television sets, measuring instruments and information terminal devices. However, CRTs are not able to be utilized in miniature electronic devices because of their relatively heavy weight and large size.
Therefore, as a substitute for CRTs, various flat display devices, such as, liquid crystal displays (LCDs), plasma display panels (PDPs), field emission displays (FEDs) and electroluminescence display (ELD), for example, have been developed. These flat display devices have the advantages of a relatively small size, light weight and low power consumption.
Among the flat panel display devices, ELDs are display devices which use electroluminescence, a phenomenon in which light is generated when a certain electric field is applied. ELDS can be classified into two categories based on their luminescence material: inorganic ELDs and organic ELDs (OELD).
OELDs are able to generate visible lights including blue lights, for example. Therefore, OELDs can display colors similar to natural colors, and are characterized by relatively high brightness and lower power consumption.
Also, OELDs have a relatively high contrast ratio of self-illumination. They are thus able to be made using ultra-thin displays, and can be fabricated in a relatively simple manner to reduce environmental pollution.
In addition, OELDs have a relatively rapid response time of a few microseconds (μs). Therefore, a moving picture can be realized more easily. There are fewer restrictions to the angle from which the picture can be viewed, and the display is stable at lower temperature.
On the other hand, an active matrix is widely used in a flat panel display. That is, a plurality of pixels are arranged in matrix form in the flat panel display and image information is supplied selectively to respective pixels through a switching device such as thin film transistors (TFTs) disposed on respective pixels.
In recent years, considerable research has been carried out on thin film transistors which use poly-crystalline silicon material. A driving unit built-in type panel has been developed which is able to improve the picture quality and reduce fabrication cost by being formed in the driving circuit in the panel of the flat panel display.
However, it is difficult to realize a driving circuit which is built in the panel because the image quality is lowered and the yield decreases rapidly due to the unevenness of the poly-crystalline silicon TFT.
The OELD of an active matrix according to a conventional current driving method will be described in detail with reference to accompanying Figures.
FIG. 1 is an exemplary view showing a rough structure of a general active matrix type OELD.
As shown in FIG. 1, an OELD panel 40 includes gate lines GL1 to GLm which cross data lines DL1 to DLn on a glass substrate 1. Pixels 10 are formed independently in square areas defined by the intersections of the gate lines GL1 to GLm and the data lines DL1 to DLn.
The pixels 10 are driven by scan signals applied through the gate lines GL1 to GLm in a gate line unit to generate light corresponding to the size of picture signals applied through the data lines DL1 to DLn.
On the OELD panel 40, a gate driving unit 20 applies a scan signal to the gate lines GL1 to GLm, and a data driving unit 30 supplies a picture signal to the data lines DL1 to DLn. The gate driving unit 20 and the data driving unit 30 are fabricated on an additional single-crystalline silicon substrate and attached on the glass substrate 1 of the panel 40 using a tape carrier package (TCP) method.
FIG. 2 shows an inner block structure of the data driving unit 30 in FIG. 1.
As shown in FIG. 2, the data driving unit 30 includes a first latch 32 which receives a control signal CS1 from a shift register 31 and sequentially samples N-bit digital picture signals DIGITAL [R, G, B] and stores the signals. A second latch 33 receives the picture signals DIGITAL [R, G, B] sampled by the first latch 32 and simultaneously transmits the picture signals DIGITAL [R, G, B] by a line pass signal LP. A voltage to current converting unit 34 converts the picture signals DIGITAL [R, G, B] transmitted from the second latch 33 into analog current values and supplies the picture signals DIGITAL [R, G, B] to the data lines DL1 to DLn of the OELD panel 40.
That is, the first latch 32 sequentially samples the picture signals DIGITAL [R, G, B] and stores the picture signals DIGITAL [R, G, B] in accordance with the control signal CS1 of the shift register 31. The second latch 33 simultaneously supplies the picture signals DIGITAL [R, G, B] stored in the first latch 32 to the voltage to current converting unit 34 in accordance with the line pass signal LP.
The voltage to current converting unit 34 converts the picture signals DIGITAL [R, G, B] into the analog current values and supplies the picture signals DIGITAL [R, G, B] to the data lines DL1 to DLn of the OELD panel 40.
If the voltage to current converting unit 34 is formed using a TFT of poly-crystalline silicon, the voltage to current converting unit 34 can be formed directly on the glass substrate 1 of the OELD panel 40.
However, in order for the voltage to current converting unit 34 to convert the picture signals DIGITAL [R, G, B] from a voltage value to an analog current value, a device such as an operational amplifier (OP-AMP) or a resistant-array besides the poly-crystalline silicon TFT should be applied. In this case, the voltage to current converting unit 34 can not be formed on the glass substrate 1, but is formed on an additional single-crystalline silicon substrate with other components of the data driving unit 30 and attached on the glass substrate 1 using the TCP method.
Most of the components of the data driving unit 30, except for the voltage to current converting unit 34, can be formed in the glass substrate 1 of the OELD panel 40 to reduce the number of integrated circuits required in the data driving unit 30. However, as described above, the data driving unit 30 of the conventional current driving method needs to be formed on an additional single-crystalline silicon substrate and attached on the glass substrate 1 of the OELD panel 40 using the TCP method. This entails a high cost of fabrication. In addition, since an additional attaching process is required, the fabrication of the OELD becomes more complex.